The early commitment of fetal neurons to the limbic cortex.

The limbic-system-associated membrane protein (LAMP) is expressed early in cerebral cortical development by migrating and postmigratory neurons in limbic regions such as the prefrontal and perirhinal cortices (Horton and Levitt, 1988), but not by nonlimbic neurons such as in the primary sensory and motor cortices. In the present study, we used LAMP expression to evaluate the timing and potential cell-lineage and environmental determinants of the commitment of cerebral cortical neurons to limbic and nonlimbic phenotypes. The cerebral wall, containing either presumptive perirhinal or sensorimotor cortex, was removed prior to [embryonic day (E) 12, 14] or just after (E17) the onset of LAMP expression, labeled with fast blue, and placed into cavities in either perirhinal or sensorimotor regions of postnatal day (P) 1 hosts. Host animals were allowed to survive until at least P10, and surviving transplanted cells were counted and evaluated for LAMP expression. Sensorimotor cortical neurons transplanted at E14 or E17, in either homotopic or heterotopic locations, did not express LAMP. In contrast, a high percentage of perirhinal cortical neurons transplanted at either E14 or E17, whether situated in sensorimotor or perirhinal regions of the host, expressed the limbic marker protein. Surprisingly, neurons from E12 donors exhibited patterns of LAMP expression that reflected their new location in the host rather than their embryonic origin. The data suggest that there is an early pliant period in which precursor and early differentiating cells remain uncommitted to a particular cortical regional phenotype. After this period, one aspect of the molecular phenotype (determined by LAMP expression) of neurons destined for sensorimotor and allo- and mesocortical regions has become immutable by environmental factors in the host.